Flash memory devices are widely used as non-volatile memory elements. Flash memories can be classified into a stacked gate flash cell type and a SONOS (polysilicon-oxy nitride-oxide-semiconductor) flash cell type. The stacked gate flash cell type has a stack structure including a floating gate, source, drain, a channel region disposed between the source and the drain, and a control gate. The SONOS flash cell type comprises a gate dielectric including an ONO-stack and a gate disposed thereon.
The basic principle of storing information in the stacked gate flash cell type is that a turn-on voltage VTH of the control gate can be shifted by depositing charges locally in the floating gate. Typically, the charges are deposited in the floating gate by applying a voltage potential to the gate and to the drain and thereby inducing tunnelling of hot carriers from the channel region through the tunnelling oxide film into the floating gate. Since the floating gate is electrically floating, the carriers are trapped in the floating gate. However, defects in the tunnelling oxide underneath the conductive floating gate may cause leakage of the device.
In comparison to the stacked gate flash cell type, a charge-trapping layer is used in the SONOS flash cell type to confine the carriers between the gate and the channel. A charge-trapping layer may, for example, comprise silicon nitride. The charge-trapping layer includes traps, effecting that the charges are not evenly distributed in the charge-trapping layer, but that the charges are localized in a certain region of the charge-trapping layer. Therefore, the SONOS flash cell type is less sensitive to defects in the tunneling oxide layer.
For achieving higher integration, the gate length of the SONOS flash cell type needs to be reduced. With decreasing gate length the performance of the SONOS flash cell is adversely affected by short channel effects that may cause malfunction of the SONOS flash cell. Therefore, the scaling properties of the conventional SONOS flash cell type is limited. What is desired is a non-volatile memory cell type with improved scaling properties.